Since the development of the first working lasers, considerable time and effort has been expended in the search for higher output laser systems. The possible applications of high power lasers are unlimited in the fields of communication, manufacturing, construction, medicine, space exploration and defense. Unfortunately many obstacles exist to the attainment of high power systems. Most lasers have low efficiency and therefore to obtain high power or high energy outputs, considerably more energy must be furnished to the system than is extracted. If this energy furnished is electrical, then the system cannot have a large average power and still be portable, as is desired in some cases. The relative size and weight of laser systems, and the availability of materials, have also introduced obstacles to their development.
The gas laser has grown out of the initial laser effort and is representative of one of the more sophisticated laser techniques which has the capability of providing very high power radiation output due primarily to the large gas handling capability characteristic of such a system and due to the large quantity of energy that can be added to the gases flowing in such systems.
A drawback in many of the high power gaseous lasers is that the material windows or reflectors which are used to isolate the medium within the laser may disintegrate under the power of the laser itself. To overcome this problem a compression-type aerodynamic window is utilized within the gaseous laser. Such a window uses supersonic flow to separate the low pressure laser cavity from the high pressure ambient environment while permitting the laser beam to be extracted from the window. One type of aerodynamic window is the single shock compression-type aerodynamic window. In the single shock aerodynamic window the shock is generated in the supersonic stream in order to support the ambient to cavity pressure difference, and the window offers an inherert advantage of good optical quality. Unfortunately, it is prone to flow separation in the capture duct and at the nozzle exit which feeds the supersonic flow.
Another type of aerodynamic window is the double shock window in which the double shock is generated by inserting a single wedge at the supersonic flow nozzle exit of the single shock window. An example of such an aerodynamic window is set forth in U.S. Pat. No. 3,654,569 issued Apr. 4, 1972. In such an arrangement the first shock is generated from the tip of the wedge and the second shock at the exit of the nozzle. In order to preserve the optical quality of the single shock window, the two shocks must not intersect within the optical path of the laser beam. As a result it requires slightly higher mass flow rates than with the single shock window. In addition, as the first two shocks coalesce, a slip-line and a weak wave may be generated from the intersection point. In addition, they may also propagate through the optical path. The optical degradation due to the slip-line and weak wave must be assessed in such window design. Furthermore, flow separation in the capture duct is more severe for the higher pressure ratio across the window, and must be solved in parallel with solving flow separation at the nozzle exit. Unfortunately, the known methods to eliminate flow separation such as boundary layer suction, blowing and wall cooling in the shock-boundary layer interaction region add mechanical complication to the entire laser system. As a result, a great need exists for the development of double shock aerodynamic window gas laser systems which alleviates the problems encountered in the past.